Acetic acid production by way of methanol carbonylation is known in the art. Generally speaking, a methanol carbonylation production line includes a reaction section, a purification section, light ends recovery and a catalyst reservoir system. In the reaction section, methanol and carbon monoxide are contacted with rhodium or iridium catalyst in a homogenous stirred liquid phase reaction medium in a reactor to produce acetic acid. Methanol is pumped to the reactor from a methanol surge tank. The process is highly efficient, having a conversion of methanol to acetic acid of typically greater than 99 percent. The reaction section also includes a flash vessel coupled to the reactor which flashes a draw stream in order to remove crude product from the reaction section. The crude product is fed to a purification section which includes generally a light ends or stripper column, a drying column, auxiliary purification and optionally a finishing column. In the process, various vent streams containing light ends, notably methyl iodide, carbon monoxide and methyl acetate are generated and fed to the light ends recovery section. These vent streams are scrubbed with a solvent to remove the light ends which are returned to the system or discarded.
In a traditional, Monsanto methanol carbonylation plant, a high pressure and low pressure absorber are included wherein acetic acid is used as the scrubber solvent. The acetic acid solvent must subsequently be stripped of light ends, usually in another purification column so that the acid is not wasted. Such columns are expensive because they must be made of a highly corrosion resistant material such as zirconium alloys and so forth. Moreover, stripping light ends from the acid requires steam and contributes to operating expense.
Nevertheless, using acetic acid as a scrubber solvent is widespread in the carbonylation art generally. See for example, U.S. Pat. No. 5,502,243 to Waller et al., entitled “Hydrocarbonylation of Dimethyl Ether”. Note the disclosure at FIG. 3, and the discussion at Cols. 8 and 9 concerning operation of an absorber 321. A cool acetic stream 323 passes downwardly through this absorber and absorbs any residual co-products and volatile catalyst components from the vent gas.
So also, there is disclosed in U.S. Pat. No. 4,241,219 to Wan, entitled “Treatment of Carbonylation Effluent”, a method of recovering volatile components by contact with a scrubbing solvent recovered from the reaction mixture in the same production line. See Col. 2, lines 15-30 wherein it is noted that acetic anhydride, ethylidene diacetate, acetic acid, or mixtures of them can be used as a vent gas scrubber solvent.
Methanol has been suggested for use as a scrubber solvent in connection with a methanol carbonylation process. In this regard, see U.S. Pat. No. 5,416,237 to Aubigne et al., entitled “Process for the Production of Acetic Acid”. It is noted in the '237 patent that noncondensables from a flash tank vapor overhead may be scrubbed countercurrently with chilled methanol. The methanol scrubber solvent residual stream is added to pure methanol and then used as feed to the reactor. See Col. 9, lines 30-42. If the reactor is not consuming the residual stream, it must be stored separately or purified again contributing to capital expense and operating costs.
Chinese Patent Application Publication No. 200410016120.7 discloses a method for recovering light components in vent gas from production of acetic acid/acetic anhydride by way of scrubbing with methanol and acetic acid. The system disclosed in the Publication No. 200410016120.7 discloses a two stage absorption arrangement wherein vent gas is treated sequentially with two different absorbents in a two stage system. Note particularly FIG. 2. Another system is seen in an industrial publication entitled “Process of 200 ktpa Methanol Low Press Oxo Synthesis AA” (SWRDICI 2006) (China). In this research publication there is shown a vent gas treatment system including a high pressure absorber as well as a low pressure absorber. Both absorbers of this system are operated utilizing methanol as a scrub fluid.
While there have been advances in the art, known methods of scrubbing vent gases in methanol carbonylation systems typically involve multiple absorber towers which are expensive to fabricate and operate. In accordance with the invention, there is provided an improved methanol carbonylation system with an absorber capable of using different solvents. The inventive system reduces both capital requirements and operating costs as compared with conventional systems.